Most people recognize sulfuric acid as a toxic, dangerous substance. Fortunately, it’s a liquid we can see and therefore, avoid. However, in its other form, hydrogen sulfide gas (H2S), poses an insidious risk because it’s invisible. In the course of a busy day, sewer workers may not be as aware as they should be of the very real dangers of H2S that collects inside wastewater conveyance pipes.

A colorless gas, H2S is toxic, flammable, and water soluble. It may also exhibit a sulfurous smell, but that can’t be depended on. And that’s a problem, because inhaling just a small amount of H2S can kill a person in just a few minutes. People don’t necessarily die from the gas itself. They mainly become overwhelmed and passing out from inhaling it, and drown in the liquid waste at the bottom of the pipe.

How It Gets There

As the organic matter in human waste breaks down, it produces H2S. The gas is also a byproduct of common industrial processes. Because it’s heavier than air, H2S tends to pool over the surface of sewage in low-lying, poorly ventilated areas. This excellent article from the September, 2017 issue of Municipal Sewer & Water magazine covers in depth how H2S forms and explains its particularly lethal character.

Sewers are perfect breeding grounds for the development of H2S, which tends to collect primarily in main lines, as opposed to service laterals. It slowly builds up along the top of the pipe, until someone opens up a manhole lid or cleanout. Suddenly, any gas trapped there is released into the air. If workers aren’t ready for it by donning their PPE, they can quickly be overcome and pass out into the pipe.

Why It’s So Dangerous

This rapid overwhelm factor makes H2S a real danger in confined space work. Workers can generally tell it’s present when they smell an odor like rotten eggs, but this isn’t always true, which makes it so dangerous. The key is to protect yourself as if H2S is always present…truly a case of “better safe than sorry.”

One issue with depending on the ability to smell the sulfurous stink of H2S is the fact that sometimes, those who have been around low levels of this gas for too long can become used to it. This can cause them to lose awareness of its presence. This is when it becomes most dangerous, because the effects of H2S are cumulative.

Potentially, H2S can damage any of the body’s physiological systems:

Cardiovascular (Heart & blood flow)

Metabolic (Energy processing)

Neurological (Nerves)

Ocular (Eyes)

Reproductive (Having children)

Respiratory (Breathing)

In low concentrations, H2S can irritate the eyes, nose and throat, potentially causing delayed respiratory effects. Greater concentrations of the gas may cause anything from vomiting to extreme respiratory distress, immediate loss of consciousness and even death.

How To Protect Yourself

Clearly, the best way to avoid becoming an H2S victim is learning to recognize it.

H2S can always be present, so just assume that it is and take proper precautions.

When removing a manhole lid, don’t hover over the top of the hole. Stand to the side, so any escaping gas can disperse. You may start feeling nauseous immediately, if you somehow breathe in a lot of it. You’ll become light-headed, then nauseous, and even develop a severe headache.

It’s especially important to stay off to the side of a manhole if you’re working alone. This way, if you do feel faint, you at least won’t fall into the hole and be completely overcome or drown.

Once you remove a manhole cover, check for this visual cue that reveals the presence of H2S: You can actually see it, as a white powdery film, along the wall of the manhole. In severe, prolonged concentrations, it will even eat the concrete. Beware of areas where you see efflorescence or rotting/crumbly material on such walls.